1. Technical Field of the Invention
The present invention relates to an apparatus and method for non-contact examination of a mirror-flat surface.
2. Description of the Prior Art
For examination of a mirror-like polished flat surface, various methods have been proposed. For example, one method is that a very narrow (several ten microns) laser light beam is projected on the mirror-flat surface and the reflected light is examined by an image sensor whereby variation of angle of the reflecting surface is calculated from the shifting of the beam incident on the image sensor, and the same measuring processes are repeated by displacing or scanning the object matter a number of times. This method can detect such shallow uneveness of up to 0.5 .mu.m, but the problem is that the detectable limit is 0.5 .mu.m and a flat large recess or a groove or a stain lying in the direction of the displacement can not be detected, the apparatus is expensive and the operation requires high skill.
Another conventional method is enlarging a narrow laser beam to enlarged parallel rays and projecting these rays on the surface, and superposing the reflected rays over the original rays and projecting them on a receiving screen, thereby forming an interference stripe pattern, and detecting unevenness of the mirror-flat surface. This method is useful in observing the surface at a glance with ease and therefore detection of scratches or the like can be easily made. However, this method has difficulty in detecting slight recesses such as of 0.5 .mu.m or shallower, moderate recess having a size of 3 to 10 mm. or stains. Also, the adjustment of the apparatus requires skilled workers and the apparatus is expensive.
In recent years, especially in the semiconductor industry, quality requirements for silicon substrate has become more and more strict as the integration density of the semiconductor devices progress higher and higher. Since internal stress of the semiconductor substrate is likely to induce defect on the mirror-polished surface thereof, the surface perfection is regarded as an important item in checking the substrate. However, in general, the surface examination is made by a skilled worker by a simple observation with his naked eyes by reflecting oblique rays or reflecting straight light source on the surface, and such examination depends much on the skill and condition of workers.
In Japan and China, mysterious mirror divine has been known which uses a simple metal-made mirror of very old tradition but having a hidden very slight intangible surface unevenness, and when sun-light or moon-light is reflected by these mirrors and projected on a far-away screen in a dark room, a mysterious pattern or character appears under a specified condition, mainly the specific distance from the mirror to the screen. The inventor tried to utilize the way of projecting the reflected light to a far-away screen for examining specific patterns corresponding to such very shallow and moderate recesses or defects on the surface. FIG. 1 schematically shows the apparatus of the above-mentioned examination after "mysterious mirror divine", where a light beam from a small light source 11 is projected through a pin hole 12 onto the mirror-polished silicon substrate 13 where a shallow and wide recess 15 exists. If there is no recess or protrusion or stain on the surface, then evenly distributed rays are reflected on the screen 14 thereby forming a simple even-brightness image of the corresponding shape to the mirror surface 131. However, since the shallow and moderate recess 15 exists on the surface 131, the recess part converges light as a concave mirror, and reflects rays as shown by the dotted lines 16. Therefore, the light image formed on the screen 14 has non-uniform distribution of brightness as shown by a curve 17. That is, in general the light image on the screen has a bright spot or bright region when the mirror face has a concave defect, and a dark spot or dark area when the mirror face has a convex defect. When the surface has parallel recess-shaped unevenness, the light image becomes a parallel shadow pattern, and one can detect such a shallow recess of 0.3 .mu.m depth and 1 to 3 mm size, but distance from the sample substrate 13 and the screen 14, as well as the distance from the light source 11 and the sample substrate 13 needs to be a large value such as 3-6 m when considering the defect recess 15 of a concave mirror of a focal length of several ten meters. Such a large apparatus is not practical for actual use in a factory. Furthermore, such a long distance induces other problems such as poor illuminance of the screen image, necessitating a large dark-room or a high power point light source, and lowering of resolution of detection due to flaring or trembling of the image, etc.
Another conventional method for detecting surface defects has been proposed to use schlieren apparatus as shown in FIG. 2, wherein a light from a small light source 11 issued through a pin-hole 12 is converged to substantially parallel rays by a condenser lens 18 and projected onto the examined surface 131 of the substrate 13. Then, light reflected by the substrate surface 131 is converged by a convex lens 19 thereby forming a focussed point 901. A knife edge 20 is provided in a manner that the focussed point just comes on the knife edge so that the rays are shadowed by the knife edge 20 and not to reach on the screen 14, thereby there is no light image formed on the screen 14. When the examined surface 131 has a concave or convex defect, the rays reflected at the defect are not converged to the focussed point 901 but form a beam with a considerable diameter around the focussed point 901, thereby passing over the knife edge 20 and projecting a pattern on the screen 14. Therefore, the convex or concave defect is detected by the forming of a pattern image on the screen. This method has a problem that among the rays scattered by the defect, only a half portion thereof is used as defect information, since half the rays reflected by the defect would be stopped by the knife edge. Therefore, when the rays reflected by the defect are bend downwards in FIG. 2, the rays only impinge on the knife edge, and no detection is made. Furthermore, the apparatus has the problem of requiring a high precision knife edge and frequent adjustment of the focussed point to be on the knife edge 20, and a considerably large size dark room.
Since the above-mentioned conventional examining methods have various problem and are not practical in an actual mass-production factory, the necessary examination has been made by means of the bare-eye observation by skilled workers, and the examination has not been efficient nor reliable. In order to raise the quality of the inspection, the examination has been a sampling test combined with thermal treatment or etching for growing the crystal imperfection for easier detection thereof.